Nano-spectroscopy of excitons in atomically thin transition metal dichalcogenides

Abstract

Excitons play a dominant role in the optoelectronic properties of atomically thin van der Waals (vdW) semiconductors. These excitons are amenable to on-demand engineering with diverse control knobs, including dielectric screening, interlayer hybridization, and moiré potentials. However, external stimuli frequently yield heterogeneous excitonic responses at the nano- and meso-scales, making their spatial characterization with conventional diffraction-limited optics a formidable task. Here, we use a scattering-type scanning near-field optical microscope (s-SNOM) to acquire exciton spectra in atomically thin transition metal dichalcogenide microcrystals with previously unattainable 20 nm resolution. Our nano-optical data revealed material- and stacking-dependent exciton spectra of MoSe2, WSe2, and their heterostructures. Furthermore, we extracted the complex dielectric function of these prototypical vdW semiconductors. s-SNOM hyperspectral images uncovered how the dielectric screening modifies excitons at length scales as short as few nanometers. This work paves the way towards understanding and manipulation of excitons in atomically thin layers at the nanoscale.

Nanoscale Optical Imaging of 2D Semiconductor Stacking Orders by Exciton‐Enhanced Second Harmonic Generation

“…The scattered signal is governed by the dielectric function of a material, thus allowing one to investigate the exciton resonance energy and oscillator strength. [ 52 ] Figure 3d shows the s‐SNOM images of the scattering amplitude demodulated at the fifth harmonic of tip‐tapping frequency, S 5 , which is proportional to local reflectance contrast within the weak resonance limit. [ 52 ] A range of incident photon energies are applied around the A exciton resonance of bilayer WSe 2 , which is expected to be at ≈1.65 eV.…”

“…[ 52 ] Figure 3d shows the s‐SNOM images of the scattering amplitude demodulated at the fifth harmonic of tip‐tapping frequency, S 5 , which is proportional to local reflectance contrast within the weak resonance limit. [ 52 ] A range of incident photon energies are applied around the A exciton resonance of bilayer WSe 2 , which is expected to be at ≈1.65 eV. Recent far‐field spectroscopy showed a variation of optical reflectance between 2H and 3R bilayers.…”

Tunable interlayer excitons and switchable interlayer trions via dynamic near-field cavity

“…However, in order to enable practical applications of TMD heterostructures using IX and IX-, several major challenges must be overcome, one of which is the large degree of spatial heterogeneity. The underlying processes, e.g., competing interactions of coupling, dephasing, and energy transfer of intra-and inter-layer excitons as well as IX-interconversion, arise at the nanoscale and cannot be understood by diffraction-limited optical approaches, calling for the near-field optical probing [14][15][16][17] . Furthermore, beyond probing, it is highly important to achieve nanoscale control of local IX and IX-properties in TMD heterostructures.…”

Resonant Coherent Raman Scattering from WSe2

“…Excitons in two-dimensional (2D) quantum materials can be optically excited and harnessed in modern optoelectronic devices. , This is true for intralayer excitons within individual atomically thin layers and also in low-dimensional heterostructures, e.g., of different transition-metal dichalcogenides (TMD), which have been shown to support long­(er)-lived interlayer excitons. , In the past few years, several methods based on photons, electrons, and photoelectrons have been developed and/or adapted to characterize excitons in space, time, and even real space–time (joint nano–femto scale). − In the realm of frequency-domain optical characterization of excitons in 2D materials, recent hyperspectral extinction as well as photoluminescence (PL) excitation measurements that are equally relevant to our present work come to mind.…”